Borax is optional and not at all necessary. If the reason you are lowering your TA and want to add borax is because you tend to have the pH rise, then lowering the TA to around 70 ppm is usually enough, but it really depends on your pool. I suggest you try a certain TA level and see where the pH wants to settle. If it is reasonably stable at 80 ppm, leave it there; if it still tends to rise, you can lower it. If it's more stable at 70 ppm or even 60 ppm, then make that your target. After you've found a reasonably stable point, then adding Borax will reduce the frequency of acid additions, though not necessarily the total amount of acid needed over time.
Note that if you have a plaster pool you may want to compensate for the low TA by targeting a somewhat higher pH (say 7.7 or 7.8) and/or increasing your Calcium Hardness (CH) level to have the Calcite Saturation Index (CSI) closer to 0 to protect plaster surfaces. For vinyl, the CH doesn't matter (well, you don't want it high since you don't want scaling, but low is fine).
